I doubt it's anything but yeast working in there. You could smell it to be sure; if you smell fermentation, and that 'wild yeast' funk, you can be sure. Wild yeasts have significant genetic variation; the most cold-tolerant of the millions of variations just replicated and took over, probably. As I understand, our best genetic analysis of lager yeast to date suggests it's a human-made chimera between two different yeast species. It's unlikely to occur in the wild, and the one (or possibly two) times it did, human selective pressure was instrumental in ensuring it survived. So no, there are probably not wild lager yeast strains....
I've had cider ferment at near-freezing temps before. Some of those wild yeast can be pretty cold-tolerant....I actually have a keg full of cider right now that fermented slowly during the course of last winter. Finished it off with lager yeast, lagered it, kegged it. Very good. Very clean, for a wild fermentation, with just a hint of 'wild yeast' funk and a bit of Czech lager aroma.
So the results of this sort of thing can be pretty nice. By contrast, I have another gallon that fermented in the fridge that, even after a few months of aging, tastes like jet fuel. Perils of wild fermentation, I guess.
